Nitriles as scintillation solvents and solutes

ABSTRACT

Benzonitrile is shown to be an efficient scintillation solvent. Although somewhat inferior to toluene, it can surpass the performance of this standard solvent if quenchers are present. Furthermore, its high dielectric constant, dipole moment and complexing ability make it suitable for the counting of polar and metallic compounds. It has the interesting property of showing an appreciable light yield in the pure state, thus allowing the counting of Alpha -emitters with no fluors added. Acetonitrile, on the other hand, is a very poor scintillation solvent, although it improves considerably on addition of naphthalene. Also, in a 40 percent mixture with benzonitrile, the efficiency is good enough for most purposes. It has been determined that other aromatic nitriles are also efficient scintillation solvents: ptolunitrile, 2,3- and 2,4-dimethylbenzonitrile and phenylacetonitrile, for example. Certain aromatic nitriles are shown to be fairly satisfactory scintillation solutes.

United States Patent Castrillon 51 June 20, 1972 [54] NITRILES ASSCINTILLATION SOLVENTS AND SOLUTES Jose P. A. Castrillon, Rio Piedras,P.R.

The United States of America as represented by the United States AtomicEnergy Commission [22] Filed: Mny6, 197i [211 Appl.No.: 140,926

[72] inventor:

[73] Assignee:

I Primary ExaminerRobert D. Edmonds Attorney-Roland A. Anderson [57]ABSTRACT Benzonitrile is shown to be an efficient scintillation solvent.Although somewhat inferior to toluene, it can surpass the performance ofthis standard solvent if quenchers are present. Furthermore, its highdielectric constant, dipole moment and complexing ability make itsuitable for the counting of polar and metallic compounds. it has theinteresting property of showing an appreciable light yield in the purestate, thus allowing the counting of a-emitters with no fluors added.Acetonitrile, on the other hand, is a very poor scintillation solvent,although it improves considerably on addition of naphthalene. Also, in a40 percent mixture with benzonitrile, the efficiency is good enough formost purposes. it has been determined that other aromatic nitriles arealso efficient scintillation solvents: p-tolunitrile, 2,3- and2,4-dimethylbenzonitrile and phenylacetonitrile, for example. Certainaromatic nitriles are shown to be fairly satisfactory scintillationsolutes.

9 Claims, 8 Drawing Figures u-Toluene 0.6 -Benzoniirile PATENTEDJUMZOI972 I 3' 67 1 455 SHEEIl 0F 6 1.0 J J5: f 8

u-Toluene Q6 o-Benzonitrile 9. some L (PPP) Fig.1.

lNVENT OR. Jose PACastriI/on TTORNE Y.

PATEmEnJun'zo 1m 7 3,671,455

' sum 2 0F 6 r.c.r. I I o-Aceton'iirile +5 11. PPo;

-Aceionitrile +17g/L PPQ 0 l I v I 1 I 0 66.7 133 200 267 333 gNaphtholene/ L mvsmon.

Jose PA. CastriIIon BY W 47% ATTORNEY.

PATENTEDJum I972 v SHEET 3 BF 6 r.c.r. u- 80% Benzonitrile Acetonitr ileA- 60% f 40% o-4o% g.S0lute L (FPO)- Fig.4

- INVENTOR. Jose RACasIri/Ion ATTORNEY.

PATENTEDJUHZO 1972 w PE ex nod :oz ac com con 02 2532 In czconcwm o n mL H 4 A R e a 6 0 m J mqo 50.82 AGIIOOJ ATTORNEY.

PATENTEflwnzo I972 c.p.m.

' 3.671.455 saw sur 6 10ml Benzonitrile lnteg rated count rote: 7,600c.p.m.

Without PPO Integrated count rate (shaded 0re0):8,400c.p.m

I warn 50mg PPO I I I l l l l I r l 1.0 3.0 5.0 7.0

DISCRIMINATOR UNITS INVENTOR. Jose PA. CasIriI/on ATTORNEY.

mrruuzs AS SCIN'IILLATION SOLVENTS AND soLuTEs BACKGROUND OF THEINVENTION This invention was made in the course of, or under, a contractwith the United States Atomic Energy Commission.

Since the early basic work of Kallman and First 1951) and Hayes andcollaborators (I955) practically no new primary solvents have beenintroduced in the field of liquid scintillation counting. Among theexceptions, isopropylbiphenyl, methyl and ethyl anthranilate,l-methyl-naphthalene, Z-ethylnaphthalene, 1,2- and1,6-dimethylnaphthalene should be mentioned. None of these, however, isin general use.

Basically, for todays operator, the choice of solvent is limited to twoclasses of compounds: aromatic hydrocarbons and ethers. The first onehas the drawback of low solubilizing power for many types of samples; inparticular, polar ones; the second. counterbalancing its lowerefficiency with the possibility it affords of counting aqueous samples,suffers the disadvantages of a generaltendency to peroxide formationthat can lead to unwanted reactions, particularly, chemiluminescentones.

There is a vast number of substances that could be called secondarysolvents which include solvents from low scintillating efficiency toeven quenchers, included in order to reduce the cost of the mixture orincrease the solubility of the sample, additives such as naphthalenethat improve the efficiency of a poor solvent, and substancesincorporated with the specific object of trapping CO dissolving tissue,etc. However, they are all based on a primary solvent belonging to oneof the two families mentioned above.

The present invention involves a third class of primary solvents, thatof nitriles,and they have certain advantages over the above-mentionedfirst two classes of solvents which will be discussed hereinbelow. Inaddition, the present invention in volves the use of certain aromaticnitriles as a new family of scintillation solutes.

SUMMARY OF THE INVENTION It is the primary object of the presentinvention to provide. for a new class of primary solvents for use inliquid scintillators for scintillation counting.

It is another object of the present invention to provide for a new classof solutes for use in liquid scintillators.

The above primary object has been accomplished in the present inventionby utilizing benzonitrile as the primary solvent in liquidscintillators, by utilizing acetonitrile (with naphthalene) as theprimary solvent, or by utilizing various mixtures of benzonitrile andacetonitrile as the primary solvent. It has been determined that otheraromatic nitriles are also efficient scintillation solvents:p-tolunitrile, 2,3- and 2,4- dimethylbenonitrile and phenylacetonitrile,for example. (P- tolunitrile is a low melting solid (29.5 C) but itsmelting point can be considerably lowered by the addition of smallquantities of other scintillation solvents). The use of benzonitrile, ifquenchers are present, can surpass the performance of the standardsolvent, toluene.

The above second object of the present invention has been accomplishedby utilizing certain aromatic nitriles as scintillation solutes in themanner to be described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS acetonitrile with addition of variousamounts of PFC.

FIG. 4 is a graph illustrating the effects of quenching with differentamounts of CCL on the logarithm of the ratio of the quenched count ratethe unquenched rate for various solvents.

FIG. 6 is a graph illustrating the effects of quenching withbenzophenone on the logarithm of the ratio of the quenched count rateover the unquenched rate for the same solvents used in FIG. 5.

FIG. 7 is a graph illustrating the relative count rates of 210,,dissolved in benzonitrile in terms of discriminator units before andafter addition of PFC.

FIG. 8 is a graph illustrating the relative counting rates of uranylnitrate dissolved in benzonitrile in terms of discriminator units beforeand after addition of PFC.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been discovered thatbenzonitrile constitutes an excellent scintillation solvent and thatwhen quenchers are present it can surpass the perfonnance of thestandard solvent toluene. The apparatus used for testing thebenzonitrile solvent to determine its usefulness as a scintillationsolvent, as well as other tests on other solvents and combination ofsolvents, was a Beckman liquid scintillation spectrometer, Model LS-II,without refrigeration, Tritium and C were counted by means of thecorresponding plug-in modules. All samples were prepared and countedunder air.

All solvents as well as the carbon tetrachloride used in the quenchingtest were spectro grade. Benzonitrile was further purified by fractionaldistillation under reduced pressure. It showed to be at least 99.99percent pure by gas chromatography. The solutes used were allscintillation grade commercial products. The reagent grade benzophenonewas recrystallized twice. It showed no contaminants on thin layerchromatography.

Three stock solutions in toluene, benzonitrile and acetonitrile wereprepared by adding 50.0 mg of C labeled phenylacetic acid to 500 ml ofeach solvent. Fifteen ml aliquots were measured out to the countingvials with a calibrated pipette and then the desired amounts ofscintillator, naphthalene or quencher were added.

The efficiencies for tritium counting were also determined byincorporating 1 mg of tritium labeled phenylacetic acid (weighed in aCahn Electrobalance to the nearest l/ I00 mg) was added to 15 ml of eachsolution.

Both the tritium and C samples contained about 80,000 d.p.m., thusensuring good counting statistics. Quenching due to the phenylaceticacid added was assumed to be negligible:

Benzonitriles efficiency for C grows on addition of solute in the samemanner as it does for the classical solvent toluene, see FIG. 1, but toa slightly lower plateau (relative counting rate, taking that in tolueneas unity, 0.98). The difference for the two solutes, PPO and PPP(p-terphenyl) are very slight and do not show in the scale of the plot.A remarkable feature of benzonitrile is that it behaves as ascintillator even when pure. The relative counting rate for C in purebenzonitrile is 0.14.

Acetonitrile, as expected, since it is not an aromatic compound, provesto be an efficient scintillation solvent; its efficiency growing more orless linearly on addition of PPO, see FIG. 2, but even for a largeamount of FPO (1.7%, 17 grams per liter), the relative counting ratedoes not surpass 0.l9. Since the efficiency of poor solvents usuallyincreases considerably on addition of naphthalene, the same expedientwas tried on acetonitrile containing 1.7% PPO, with the results shown inFIG. 3. The relative counting rate does, indeed, increase, and does notyet level of? at about 200 grams naphthalene per liter and a relativecount rate of 0.83. Addition of naphthalene to a less concentratedsolution, containing only 0.5% (5 grams/liter), shows in FIG. 3 that inthis case the same efficiency can be reached with much less naphthalene.

Plots that represent the influence of addition of PFC on mixtures ofbenzonitrile-acetonitrile are shown in FIG. 4. As expected, theefficiency decreases with increasing content of acetonitrile. The regionof 40-60 percent content of acetonitrile seems to afford a goodcompromise for the counting of aqueous samples.

The relative counting rates for tritium in different solvents, allcontaining the same concentration of PPO (0.5 percent) and POPOP (0.05percent), are listed in Table l. POPOP may be identified as:l,4-bis-2-(S-phenyl-oxazolyl)-benzene].

(a) PP? as solute. instead of PFC.

Here, what was not obvious for C measurements, is clearly shown, thatis, that the light yield in benzonitrile is lower than intoluene. Still,it is comparable to that of the widely used mixture:dioxane-naphthalene. The mixture 60 percent benzonitrile-40 percentacetonitrile, still provides a reasonable efiiciency, while theperformance of acetonitrilenaphthalene is very poor.

The effect of two strong quenchers, benzophenone and carbontetrachloride, was studied for C counting in toluene, benzonitrile andacetonitrile-naphthalene scintillation solutions. The solutionscontained 5 grams PPO per liter of solvent, with the addition of 200grams naphthalene in the case of acetonitrile.

The counting window was established by raising the lower discriminatoruntil the counting rate of the unquenched sample was reduced to half.The logarithm of the ratio of the quenched count rate over theunquenched rate was then plotted versus the molar concentration of thequencher. Although the experimental points do not strictly fall onstraight lines, these can be approximately drawn to visualize thedifferent degrees of quenching. From FIGS. 5 and 6, it can be seen thatbenzonitrile is much less sensitive to the effect of both quenchers.Since benzonitrile is almost as efficient'as toluene, these results showthat in the presence of a quencher, benzonitrile can show an efiiciencyconsiderably higher than that of toluene. For example, at aconcentration of CCl, of 433 X l0 m/l, for the same activity of C, thecounting rate in benzonitrile is 5.7 times that in toluene; at aconcentration of benzophenone of l 17 X m/l, 6.1 times. The lessersensitivity to quenching of benzonitrile compared to toluene, can bedue, at least in part, to its higher viscosity.

Although the solubility of water in benzonitrile is much higher than intoluene, it is still very small. The benzonitrile scintillation solutionused in this test dissolves approximately 0.5 percent water without anynoticeable efficiency loss of C, or even tritium counting. Largeramounts of water, up to 1 percent and more, can be counted inbenzonitrile for water disperses very easily on vigorous shaking givingstable suspensions.

In the scintillation solution obtained from 60% benzonitrile- 40 percentacetonitrile. the solubility of water is approximately 3.3 percent.Excess water disperses more readily then in pure benzonitrile, but thesuspension is less stable.

Since peroxides are not formed in nitriles by standing, one could expectthe absence of the chemiluminescent reactions, due to peroxides, whichare so frequent when ethers are used as solvents. Nevertheless, ifstrong oxidizing agents are contained in the sample, chemiluminescentreactions may occur. The addition to l0 ml samples of benzonitrile of 1mg mchloroperbenzoic acid or iodoso benzene diacetate gave countingrates in the order of l0,000 per minute, presumably due to nitrile oxideformation.

Since benzonitrile is a scintillator per se," albeit a not veryefficient one, it allows the counting of energetic particles without theaddition of a fluor. This property plus a good solubilizing ability forheavy metal ions, via complex formation, makes it particularlyconvenient for a counting.

"Po emits 01 particles of 5.3 MeV and no Bs, decaying into stable Pb.Therefore, except for a very slight proportion of 'ys, it serves as apure 0: source. A trace amount in the form of polonium nitrate wasdissolved in 10 ml benzonitrile, counted and the spectrum ran in termsof discriminator units, taking successive 2 percent windows, before andafter addition of 50 mg PPO, see FIG. 7. in the presence of PPO, the apeak is strongly displaced to the high energy end, but the integratedcount rate remains the same, suggesting that the a particles are countedwith near percent efficiency in pure benzonitrile. A second peak in thelow energy end appears, and it is probably due to contamination with anemitter of weak ,Bor X rays.

Entirely analogous results are obtained with Am, except that in thiscase the low energy peak that the addition of PFC reveals, can beattributed to the conversion electrons and X rays also present in theradiation of this isotope.

The results obtained with uranyl nitrate dissolved in benzonitrile areshown in H6. 8. It should be remembered that it is a complex emitterincluding members of the chain between U and "U, and that energetic Bsmay be superimposed with the a portion of the spectrum. In the lowenergy peak, weak betas plus internal conversion of electrons may besuperimposed. The efficiency is not very high, but the fact the solutionis markedly yellow must be kept in mind.

The good and other features of benzonitrile as a scintillation solventin comparison with toluene and dioxane will now be summarized.Benzonitriles efficiency is lower than that of toluene, and higher thanfor pure dioxane. It is, in fact, comparable to the widely usedcombination dioxane-naphthalene; this means that, except for the mostweak B emitters, no significant reduction in the counting rate will beobserved if used instead of toluene. Furthermore, in the case ofquenched samples it may be more efficient than toluene.

Benzonitrile is, of course, highly toxic, but this should not be aserious disadvantage, since adequate precautions (well ventilated hoods,avoidance of skin contact, etc.) can be easily taken.

Although the melting point, l 3 C, is much higher than in the case oftoluene, it still leaves ample room for refrigerated counting and isconsiderably lower thanthat of dioxane.

The occurrence of chemiluminescent phenomena, in the presence ofoxidants, is a disadvantage with regard to toluene which is shared byethers. In fact, since after standing in air, ethers and dioxane, inparticular, will contain peroxides, this nuisance can be expected to bemore frequent in ether based solvents than in benzonitrile.

Although pure benzonitrile is higher in cost than other solvents, it maynevertheless be the solvent of choice. lt has a high dipole moment (4.02in benzene at 20 C), and a high dielectric constant (26.3; foracetonitrile it is even higher; 36.4) which makes it an excellentsolvent, particularly for metallic salts. To mention only a fewexamples, silver salts are, in general, soluble in benzonitrile; silvernitrate, in particular, dissolving at the rate of grams per 100 grams ofbenzonitrile; and many other salts of different cations are also solublein benzonitrile: (CH COO) Zn, FeCl LiCl HgCl SbCI and CuCl for example.In general, acetonitrile is a better solvent and, therefore, thebenzonitrile-acetonitrile mixtures and even the acetonitrile-naphthalenesystem, would be convenient in some liquid scintillators.

Concerning the possibility of Cherenkoff counting of energetic betas inbenzonitrile, since its refractive index is 1.5289 at 20 C, thethreshold for B detection would be KeV. This means that for B'sexceeding this energy, Cherenkoff photon will add to the fluorescencephotons, facilitating counting in pure benzonitrile.

It should be understood that the present invention is not limited to thenitriles discussed above. It has been determined that other aromaticnitriles, as listed in the Summary of the Invention" above are alsoefficient scintillation solvents.

From the above discussions, it can be seen that benzonitrile is anefficient scintillation solvent, and that it has the unusual property ofbehaving as a moderate scintillator even when pure. This would seem toindicate the possibility that aromatic nitriles of suitable structurescould function as scintillation solutes. The fluorescence of aromaticnitriles is well known and since the introduction of a cyanosubstitutent, in general, enhances fluorescence, it can be expected thatsome of these compounds, in particular, those derived from knownscintillators will be useful as either primary or secondary solutes.

The scintillation spectrum of Cd, dissolved in the form of2-ethylhexanoate, in deaerated toluene solution has been determined bymeans of a Beckman liquid scintillation spectrometer, Model LS-ll, usinga series of aromatic nitriles as scintillation solutes. At the sameconcentrations used later to determine tritium efficiencies, fourmembers of the series: 4 ,4 -dicyanobiphenyl, 9, l O-dicyanoanthracene,4- cyanoterphenyl, and 4,4-dicyanoterphenyl showed the peakcorresponding to the 84 KeV conversion electrons of Cd at the same orhigher position in the discriminator as done by the solute PPP. However,a certain degree of smearing in the spectra was evident.

The efliciency of these compounds in aerated toluene solution fortritium counting relative to PPP was also determined. This provides astringent test of more practical value; the

results are shown in the following Table ll:

TABLE I] Concentration Effic- Substance Color (Mol/l) iency P-terphenylcolorless 0.0200 l.0O

(PPP) 2,5-Diphenyloxazole colorless 0.0200 l.0l

PPO) 9 l0-Dicyanoanthracene yellow 0.0029 0.49

(CAC 4,4'-Dic vanobiphenyl colorless 0.0200 0.83

(CBC) 4-Cyunoterphenyl colorless 0.0078 0.89

(CTP) 4 4'-Dicyanoterphenyl colorless 0.0012 0.57

(CTPC) Compounds CAC, C! P and CT PC were tested at lower concentrationsbecause of poor solubility. Although none of the nitriles testedsurpasses the standard solutes, PPO and PPP, they show fairly highefficiencies and could be used for tritium counting. The result obtainedwith the very low concentration CT PC is particularly striking. For Cwith fls of considerably higher energy, the eificiencies of CBC, CTP andCTPC are practically the same as for PPO and PPP, the comparison beingmade at the concentrations already indicated.

This invention has been described by way of illustration rather thanlimitation and it should be apparent that it is equally applicable infields other than those described.

What is claimed is:

1. An improved liquid scintillator mixture consisting essentially of asolvent selected from the group consisting of benzonitrile,acetonitrile, p-tolunitrile, 2,3-dimethylbenzonitrile,2,4-dimethylbenzonitrile, and phenylacetonitrile, and a primary soluteselected from the group consisting of p-terphenyl and2,5-diphenyloxazole of a selected amount in the range from 1 gram to 7grams per liter of solvent.

2. The scintillator set forth in claim 1, wherein said selected solventis benzonitrile, and said selected solute is 2,5-diphenyloxazole.

3. The scintillator set forth in claim 1, wherein said selected solventis benzonitrile, and said selected solute is p-terphenyl.

4. The scintillator set forth in claim 1, wherein said solvent consistsof a mixture of benzonitrile and acetonitrile with the content of saidacetonitrile in said mixture being of a selected amount in the rangefrom 40-60 percent and said selected solute is 2,5-diphenyloxazole.

5. The scintillator set forth in claim 1, wherein said selected solventis acetonitrile, said mixture further including naphthalene of aselected amount in the range from 60 grams to 200 grams per liter.

6. The scintillator set forth in claim 1, wherein said selected solventis p-tolunitrile.

7. The scintillator set forth in claim 1, wherein said selected solventis 2,3-dimethylbenzonitrile.

8. The scintillator set forth in claim I, wherein said selected solventis 2,4-dimethylbenzonitrile.

9. The scintillator set forth in claim 1, wherein said selected solventis phenylacetonitrile.

II i i

2. The scintillator set forth in claim 1, wherein said selected solventis benzonitrile, and said selected solute is 2,5-diphenyloxazole.
 3. Thescintillator set forth in claim 1, wherein said selected solvent isbenzonitrile, and said selected solute is p-terphenyl.
 4. Thescintillator set forth in claim 1, wherein said solvent consists of amixture of benzonitrile and acetonitrile with the content of saidacetonitrile in said mixture being of a selected amount in the rangefrom 40-60 percent and said selected solute is 2,5-diphenyloxazole. 5.The scintillator set forth in claim 1, wherein said selected solvent isacetonitrile, said mixture further including naphthalene of a selectedamount in the range from 60 grams to 200 grams per liter.
 6. Thescintillator set forth in claim 1, wherein said selected solvent isp-tolunitrile.
 7. The scintillator set forth in claim 1, wherein saidselected solvent is 2,3-dimethylbenzonitrile.
 8. The scintillator setforth in claim 1, wherein said selected solvent is2,4-dimethylbenzonitrile.
 9. The scintillator set forth in claim 1,wherein said selected solvent is phenylacetonitrile.